NOx, Hg, and SO2 removal using ammonia

Abstract

A process and apparatus for removing SO2, NO, and NO2 from a gas stream having the steps of oxidizing a portion of the NO in the flue gas stream to NO2, scrubbing the SO2, NO, and NO2 with an ammonia scrubbing solution, and removing any ammonia aerosols generated by the scrubbing in a wet electrostatic precipitator. The process can also remove Hg by oxidizing it to HgO and removing it in the wet electrostatic precipitator. Ammonium sulfate, a valuable fertilizer, can be withdrawn from the scrubbing solution.

Description

BACKGROUND OF INVENTION

[0001] a. Field of the Invention

[0002] This invention relates to methods and apparatuses for removing NOx and SO2 from a gas stream.

[0003] b. Description of the Related Art

[0004] Fossil fuels are burned in many industrial processes. Electric power producers, for example, burn large quantities of coal, oil, and natural gas. Sulfur dioxide (“SO2”), nitrogen oxide (“NO”), and nitrogen dioxide (“NO2”) are some of the unwanted byproducts of burning any type of fossil fuel. Mercury (“Hg”) is often also found in fossil fuels. These byproducts are known to have serious negative health effects on people, animals, and plants, and a great deal of research has been done to find a way to economically remove them from flue gas streams before they enter the atmosphere.

[0005] SO2 is often removed from gas streams (“desulfurization”) by scrubbing the gas with an aqueous ammonium sulfate solution containing ammonia. Examples of this process are disclosed in U.S. Pat. Nos. 4,690,807,...

Examples

example 1

[0039]An absorption test was done for the scrubbing step of the process of the present invention, with a solution that was 1% w / w SO32− (“sulfite”), 6% w / w SO42− (“sulfate”), and 2.5% S2O32− (“thiosulfate”) in a packed column that was 18 inches high and 1.5 inches in diameter. The column was packed with ¼ inch glass RASCHIG rings. The simulated flue gas at the inlet of the column contained 13% v / v moisture, 6% v / v O2 and the simulated flue gas pollutants listed in the table. There was continuous addition of NH3 and (NH4)2S2O3 to maintain a pH of 6.8 and a thiosulfate concentration of 2.5% w / w. The residence time in the column was 1.8 sec with an L / G ratio of 25 gpm / kacfm.

[0040]The table shows the concentrations of NO, NO2, and SO2 at the inlet and outlet of the test system.

[0041]

TABLE 1Scrubbing Step AloneSystem InletSystem OutletNO (ppmv)204NO2 (ppmv)25036SO2 (ppmv)13702

example 2

[0042]An absorption test was done for the scrubbing step of the process of the present invention starting with water and a flue gas stream consisting of 13% v / v moisture, 17 ppmv NO, 267 ppmv NO2, 1360 ppmv SO2, 6% v / v O2 and balance N2. Ammonia and ammonium thiosulfate were added to maintain a pH of 6.8 and a thiosulfate concentration of 2.5%, and the concentrations of sulfite and sulfate in the system were allowed to build to steady state. The NOx removal rate was 80% w / w at concentrations of SO32−, SO42− and S2O32− of 0.7% w / w, 2.5% w / w, and 0.5% w / w respectively.

example 3

[0043]Tests were conducted in a laboratory test facility for the NO oxidizing, scrubbing, and aerosol removal steps of the process of the present invention. The equipment consisted of a simulated flue gas delivery system, a coaxial cylinder DBD reactor, a packed column scrubber and a tubular WESP. The following is an example of data obtained in the lab test facility.

[0044]Simulated flue gas was delivered to the DBD reactor at a flow rate of 14 scfm, a temperature of 290° F. and with the following composition: 6.2% v / v O2, 14.2% v / v CO2, 8.2% v / v H2O, 20 ppmv CO, 250 ppmv C2H4, 1740 ppmv SO2, and 259 ppmv NOx. Gas velocity through the discharge reactor was 50 ft / sec with discharge power level of 140 watts.

[0045]Gas from the discharge reactor entered a 4″ID packed column scrubber, packed with ½″INTALOX saddles to a depth of 4 feet. Liquid was introduced at the top of the scrubber at a flow rate of 0.33 gpm (L / G=20 gpm / kacfm). Aqueous ammonia was added to and effluent liquid removed fr...

BACKGROUND OF INVENTION

[0001] a. Field of the Invention

[0002] This invention relates to methods and apparatuses for removing NOx and SO2 from a gas stream.

[0003] b. Description of the Related Art

[0004] Fossil fuels are burned in many industrial processes. Electric power producers, for example, burn large quantities of coal, oil, and natural gas. Sulfur dioxide (“SO2”), nitrogen oxide (“NO”), and nitrogen dioxide (“NO2”) are some of the unwanted byproducts of burning any type of fossil fuel. Mercury (“Hg”) is often also found in fossil fuels. These byproducts are known to have serious negative health effects on people, animals, and plants, and a great deal of research has been done to find a way to economically remove them from flue gas streams before they enter the atmosphere.

[0005] SO2 is often removed from gas streams (“desulfurization”) by scrubbing the gas with an aqueous ammonium sulfate solution containing ammonia. Examples of this process are disclosed in U.S. Pat. Nos. 4,690,807,...

Examples

example 1

[0039]An absorption test was done for the scrubbing step of the process of the present invention, with a solution that was 1% w / w SO32− (“sulfite”), 6% w / w SO42− (“sulfate”), and 2.5% S2O32− (“thiosulfate”) in a packed column that was 18 inches high and 1.5 inches in diameter. The column was packed with ¼ inch glass RASCHIG rings. The simulated flue gas at the inlet of the column contained 13% v / v moisture, 6% v / v O2 and the simulated flue gas pollutants listed in the table. There was continuous addition of NH3 and (NH4)2S2O3 to maintain a pH of 6.8 and a thiosulfate concentration of 2.5% w / w. The residence time in the column was 1.8 sec with an L / G ratio of 25 gpm / kacfm.

[0040]The table shows the concentrations of NO, NO2, and SO2 at the inlet and outlet of the test system.

[0041]

TABLE 1Scrubbing Step AloneSystem InletSystem OutletNO (ppmv)204NO2 (ppmv)25036SO2 (ppmv)13702

example 2

[0042]An absorption test was done for the scrubbing step of the process of the present invention starting with water and a flue gas stream consisting of 13% v / v moisture, 17 ppmv NO, 267 ppmv NO2, 1360 ppmv SO2, 6% v / v O2 and balance N2. Ammonia and ammonium thiosulfate were added to maintain a pH of 6.8 and a thiosulfate concentration of 2.5%, and the concentrations of sulfite and sulfate in the system were allowed to build to steady state. The NOx removal rate was 80% w / w at concentrations of SO32−, SO42− and S2O32− of 0.7% w / w, 2.5% w / w, and 0.5% w / w respectively.

example 3

[0043]Tests were conducted in a laboratory test facility for the NO oxidizing, scrubbing, and aerosol removal steps of the process of the present invention. The equipment consisted of a simulated flue gas delivery system, a coaxial cylinder DBD reactor, a packed column scrubber and a tubular WESP. The following is an example of data obtained in the lab test facility.

[0044]Simulated flue gas was delivered to the DBD reactor at a flow rate of 14 scfm, a temperature of 290° F. and with the following composition: 6.2% v / v O2, 14.2% v / v CO2, 8.2% v / v H2O, 20 ppmv CO, 250 ppmv C2H4, 1740 ppmv SO2, and 259 ppmv NOx. Gas velocity through the discharge reactor was 50 ft / sec with discharge power level of 140 watts.

[0045]Gas from the discharge reactor entered a 4″ID packed column scrubber, packed with ½″INTALOX saddles to a depth of 4 feet. Liquid was introduced at the top of the scrubber at a flow rate of 0.33 gpm (L / G=20 gpm / kacfm). Aqueous ammonia was added to and effluent liquid removed fr...

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